Currently, regulation of exhaust gas on an internal combustion engine (engine) for automobiles and industries using an internal combustion engine has become strict year by year. In addition, as one of measures against global warming, introduction of strict fuel economy regulation has been examined latterly. Each automaker has proceeded with research and development relating to various devices for improving engine performance such as exhaust gas performance, fuel economy and the like in order to respond to these regulations.
The internal combustion engines are roughly divided into a gasoline engine and a diesel engine depending on a difference in a fuel to be used. Since the gasoline engine has lower heat efficiency and poorer fuel efficiency as compared with the diesel engine, research and development for improvement of heat efficiency have been made in order to improve the problems, and development of various devices for reduction of engine friction by making intake/exhaust valve mechanisms and auxiliary machines variable and the like has rapidly progressed.
On the other hand, in the diesel engine, research and development on devices such as high-pressure injection, high pressure charging and the like have rapidly progressed latterly, but the research is delayed as compared with the gasoline engine in the area of fuel efficiency improvement. Moreover, a difference in fuel efficiency from the gasoline engine has been increasingly narrowing, and in view of a prospective introduction of fuel economy regulation in addition to the strict exhaust gas regulation, research and development for improving the fuel efficiency of the diesel engine is important.
Methods for improving the fuel efficiency of the diesel engine are roughly divided into two types. One of them is a method of reducing fuel use by improving heat efficiency through improvement of combustion in an engine cylinder. Regarding this method, since heat efficiency of the diesel engine has been already at an extremely high level as compared with that of the gasoline engine, even if the heat efficiency could be improved, much better fuel efficiency cannot be expected, and further improvement of the heat efficiency is extremely difficult.
Another method is to reduce friction of an engine main body and an engine auxiliary machine, which has been actively employed in the gasoline engine, and development of similar devices relating to this method has progressed also in the diesel engine. There are various types of friction, but almost a half thereof is caused by a pumping loss of an engine.
The pumping loss of the engine will be described. An illustrated work of an engine is expressed in general by a “P-V diagram” of a pressure in an engine cylinder with an engine cylinder capacity (cm3: lateral axis) and an engine cylinder pressure (MPa: vertical axis). The pumping loss of an engine is a region located on a lower side of this “P-V diagram”, and an area of this region becomes a loss workload. That is, friction generated by an engine during an exhaust stroke in which a piston is raised after combustion of the engine, an exhaust valve is opened, and an exhaust gas in the cylinder is pushed out and during an intake stroke in which the piston is lowered, an intake valve is opened, and new air is introduced into the cylinder, is the pumping loss.
This pumping loss occurs similarly in the gasoline engine and the diesel engine, but in the case of the gasoline engine, a mixture ratio between a fuel and an air amount needs to be made constant, unlike the diesel engine. Thus, an intake throttle valve needs to be provided in an intake line in the gasoline engine, and the pumping loss becomes larger due to its influence than in the case of the diesel engine. However, in the recent gasoline engines, there is a tendency of removing this intake throttle by changing an operating amount of the intake valve to adjust a mixture ratio between the fuel and the air amount. As a result, the pumping loss of the gasoline engine is getting closer to that of the diesel engine in recent years.
Moreover, in a part of vehicles equipped with the gasoline engine, a reduced-cylinder operation system for improving fuel efficiency by reducing the pumping loss has been already employed. In this reduced-cylinder operation system, a part of intake/exhaust valves are stopped depending on an operation state of an engine so as to reduce the number of operating cylinders, and fuel efficiency is improved by reducing the pumping loss as the entire cylinder. In this reduced-cylinder operation system, by stopping the intake/exhaust valves, the line in the above “P-V diagram” becomes substantially one line, and an area of the pumping loss becomes substantially zero. Thus, drastic improvement of the fuel efficiency of the engine can be realized.
In this method of stopping the intake/exhaust valves, various proposals have been already made such as a use of an electromagnetic valve, a structure in which a rocker arm is divided into two parts, and a connecting pin is made to slide by a hydraulic pressure and the like to make a valve lose motion and the like, and they have been already employed in mass production in gasoline engines.
However, they have not employed in mass production in the diesel engines. The reason for that is a drop in supercharging amount. Unlike gasoline engines, if a large amount of air excess ratio is not ensured with respect to a theoretical air amount required by a fuel inputted into the engine, combustion of the engine deteriorates in the diesel engine. Thus, in the current diesel engine, it is a general practice to perform turbo-charging by compressing and turbo-charging air by a turbocharger operated by using exhaust energy of an exhaust gas. Thus, if the reduced-cylinder operation system is employed for the diesel engine, the pumping loss is drastically reduced, but to the contrary, a large decrease of a flow rate of the exhaust gas shifts an operating point of the turbocharger, which largely changes an operation region of this turbo, and as a result, the turbocharger cannot work sufficiently, and a supercharging amount of the engine drops. As a result, combustion of the engine deteriorates, fuel efficiency becomes poor, and comprehensive fuel efficiency performance deteriorates.
The large drop of the supercharging amount will be described in more detail. Regarding a stop cylinder in the reduced-cylinder operation system, due to an influence of an increase in vibration when a number of engine cylinders in activating are decreased, it is a general practice that two cylinders are stopped in the case of four cylinders and three cylinders are stopped in the case of six cylinders. For example, if a half of the cylinders are stopped in a light load when the turbocharger is not sufficiently operating, an exhaust gas flow rate of the engine also becomes approximately a half, and a large shift is made to the left on a turbo map. When this is considered on a turbo compressor map expressed by a gas flow rate (lateral axis) and a pressure ratio (vertical axis) between an intake and an outlet of the turbocharger, the condition enters into the lowest and the leftmost position in a region where it is not operating as the turbocharger. Therefore, even if a fuel is slightly increased and an engine load is raised from this state, sufficient supercharging cannot be obtained from the turbocharger, and thus, fuel efficiency improvement of the engine cannot be realized.
In relation with that, as described in Japanese patent application Kokai publication No. 2010-223040 (Patent Document 1), for example, in order to be able to continue exhaust gas recirculation without causing a trouble in a turbocharger even if the reduced-cylinder operation is performed in a turbo-charging engine, a method and a device for exhaust gas recirculation of a turbo-charging engine are proposed by providing a low-pressure exhaust gas recirculation (“EGR”) loop and a high-pressure EGR loop, recirculating the exhaust gas by selecting the low-pressure EGR loop during a reduced-cylinder operation so that the full amount of the exhaust gas passes through the turbocharger, while by recirculating the exhaust gas by selecting the high-pressure EGR loop or the both of the high-pressure EGR loop and the low-pressure EGR loop during a normal operation when the reduced-cylinder operation is not performed.
However, with this method and device, though a drop in the turbo-charging amount by the EGR can be prevented by holding the supercharging amount high by using the low-pressure EGR rather than the high-pressure EGR during the reduced-cylinder operation, it does not reach the supercharging amount during a normal time when operating on a full-cylinder basis, and also, since the supercharging amount lowers in a reduced-cylinder operation state even if the EGR is not used, the turbo operating point lowers and the engine fuel efficiency deteriorates, and thus, a problem is caused that a drop in the supercharging amount of turbo-charging by the reduced-cylinder operation cannot be prevented. Moreover, since the exhaust gas is made to pass through the turbocharger and an intercooler in the low-pressure EGR, a problem of corrosion such as oxidation occurs in components of those paths, which leads to a problem that completion as a system is extremely difficult.
Moreover, as described in Japanese patent application Kokai publication No. 2006-177191 (Patent Document 2), for example, in order to improve shortage of a supercharging amount of a cylinder operating by employment of the reduced cylinder system, an engine in which a plurality of turbochargers are attached by employing small-sized turbochargers with respect to a base turbocharger is proposed.
However, this engine has a problem that engine performance or particularly, fuel efficiency is deteriorated in a full-load operation region other than the reduced cylinder operation region. That is, a large-sized diesel engine has fuel efficiency better than that of a small-sized diesel engine in general due to a great influence by a difference in turbo efficiency, and this turbo efficiency is extremely higher in the large-sized turbochargers than in the small-sized turbochargers in general, and moreover, its region is also large. Therefore, in this engine, though the shortage in the supercharging amount generated in the reduced cylinder operation can be improved, the engine performance or particularly, fuel efficiency is deteriorated by attachment of small-sized turbochargers with respect to the engine performance based on the full-cylinder operation with a full load.
FIG. 7 illustrates a graph of “engine speed and engine output” and a graph of “compressor map (air flow rate and pressure ratio)” created on the basis of an image of a system of a single-stage supercharging type normal engine. Moreover, FIG. 8 illustrates a graph of “engine speed and engine output” and a graph of “compressor map (air flow rate and pressure ratio)” created on the basis of an image of a system of an engine and two units of small-sized turbochargers performing the reduced-cylinder operation of Patent Document 2.
In the case of this Patent Document 2, when a small-sized turbocharger for a reduced cylinder is used, compressor efficiency (illustrated by a contour ellipse indicated by reference numerals 60, 70 and the like) rises to some degree in a region where a load is low as indicated by an A-point, for example, and supercharging efficiency can be improved. However, in a region where a load is high as illustrated by a B-point exceeding a reduced-cylinder operation region, it falls lower than that of the normal engine in FIG. 7 to the contrary.
Of course, regarding a turbocharger, a region to be used can be changed depending on a selection of specification or on matching, but as described above, since a region with a good efficiency which is a merit of employment of a small-sized turbocharger is small, and since the efficiency of the small-sized turbocharger is lower than that of the large-sized turbocharger, the efficiency cannot be improved in a region where a load is high. Thus, its employment is difficult since fuel efficiency becomes poor in a region with a high load which is frequently used in commercial vehicles and the like.
In order to solve the above described problems, a system of “reduced cylinder+plurality of small-sized turbochargers +low-pressure stage large-sized turbocharger” can be considered. However, in order to realize this system, it is important to solve a problem which is a hindrance in obtaining improved engine performance, that is, one of the small-sized turbochargers which is operating during a reduced-cylinder operation cannot circulate and pressurize charged air to the other small-sized turbocharger which is stopped during the reduced-cylinder operation.
As improvement for this problem, as described in Japanese patent application Kokai publication No. 2011-231683 (Patent Document 3), for example, an internal combustion engine is proposed. The engine is provided with a two-stage supercharging system, in which a high-pressure stage turbocharger is arranged in each of a plurality of exhaust passages, and an exhaust gas flowing out of these high-pressure stage turbochargers is introduced into a low-pressure stage turbocharger, and a three-way valve is attached at a portion where intake passages from the both high-pressure stage turbochargers merge.
However, this internal combustion engine has two the problems of a structure of the three-way valve and deterioration of turbo durability caused by a mounting position.
Regarding the problem of the structure of the three-way valve, there is no detailed description on the three-way valve used in the above internal combustion engine, and it is presumed from FIG. 1 in Patent Document 3 that the three-way valve has a mechanism to be automatically opened/closed by a negative pressure, but an engine intake side (intercooler) of this three-way valve requires an opening area larger than a total area of the respective areas of the high-pressure stage turbochargers, and if the area is smaller than that, a pressure loss occurs during an engine full-load operation, and a problem of deterioration of engine performance is caused. If such large opening area is to be ensured, the size of the three-way valve increases, and the system becomes extremely large and impractical.
In FIG. 1 of Patent Document 3, the valve of the three-way valve is half-open, but the opening area of this valve portion is extremely small. In this three-way valve, by means of a pressure difference generated during a return from the reduced-cylinder operation to a base full-cylinder operation or during the respective engine combustions or turbo performances are varied, an action to close the valve of the three-way valve to one side works, and as a result, an engine pumping loss deteriorates due to a rise of an exhaust pressure in the engine cylinder of a group with a small pressure, and thus, the engine performance such as exhaust gas performance, fuel efficiency performance and the like deteriorate. Thus, the performance of the system cannot be improved.
Regarding the problem of deterioration of turbo durability caused by a mounting position, the mounting spot of the three-way valve being at an outlet of the high-pressure stage turbocharger is the problem. That is, assuming that, while each of the high-pressure stage turbochargers is operating close to the maximum rotation during an engine operation at a full load, the operation suddenly enters the reduced-cylinder operation region,-a fuel is stopped to one of the cylinder groups, and the intake/exhaust valves are also stopped at the same time, while the other cylinder group performs the reduced-cylinder operation, but since the exhaust gas energy obtained by the high-pressure stage turbocharger on this reduced-cylinder operation side decreases, a pressure of the compressed air decreases, whereby the three-way valve is rapidly closed.
However, since the high-pressure stage turbocharger is still rotating at a high speed at this time, a state in which a flow rate is not gained even though supercharging is performed occurs. By plotting this state on a compressor map illustrated in FIG. 9, the pressure ratio is extremely high, while the flow rate is close to zero. That is, in transition from a C-point to a D-point in FIG. 9, as illustrated in the compressor map of the turbocharger of a cylinder stopped during the reduced cylinder operation, the high-pressure stage turbocharger for a stopped cylinder has its outlet rapidly closed by the pressure difference, and the air flow rate falls to zero. However, since this high-pressure stage turbocharger is still rotating, the state exceeds a surge line without fail at the moment when the outlet is closed. Thus, the turbocharger would be broken unless a special measure is taken.
Therefore, the above system of “reduced cylinder+plurality of small-sized turbochargers +low-pressure stage large-sized turbocharger” provided with the three-way valve does not fundamentally or essentially improve the problems in obtaining-a fuel efficiency improvement effect, and mass production is difficult.
That is, as described above, when a supercharging system having one or a plurality of turbochargers in a diesel engine of an internal combustion engine is combined with a reduced-cylinder operation system having a valve stopping mechanism, a problem occurs that an engine exhaust gas of the engine performance deteriorates due to a lowered supercharging amount, and moreover, an effect of improvement of engine fuel efficiency which is a target cannot be sufficiently obtained. Specifically, a state becomes only a light-load operation state in which a region where fuel efficiency improvement can be extremely small. On the other hand, mass production has been already realized in the gasoline engine, and the effect of fuel efficiency improvement that can be obtained by lowered engine pumping realized by introduction of the reduced-cylinder operation system is obvious, but an influence of the above described problems is great in the diesel engine in which an air excess ratio is important.